1. Field of the Invention
The present invention relates to power supply devices, and particularly to a plugtype power supply unit having function of an AC/DC power supply and structural configuration thereof complies with American Standard NEMA 1-15P and can be inserted into the American standard receptacle.
2. Description of the Prior Art
Nowadays, most electronic products, especially portable consumer electronic products, such as mobile phones, mp3 players, CD players and so on, need low-voltage power, often from 3 v to 5 v, and have low power consumption from 1 w to 20 w. Conventionally a power supply unit with an AC/DC converting function is needed to achieve a low voltage. As electronic technologies evolve, electronic products are becoming smaller and smaller, being portable and smart. Accordingly, the dimensions and structural configurations of the power supply units have to meet more rigorous specifications.
Over time there have been several inventions focused on integrating AC/DC converters into the mains plug used for the mains power supply. All over the world different standards for electronic devices exist, and AC/DC power supplies must comply with these standards. One set of such standards are the IEC standards. IEC 60950 describes the minimum requirements for electrical equipment with regard to isolation and minimum distances between components with different voltage levels. IEC 61000 describes minimum requirements for electromagnetic compatibility (EMC). Usually, some standards are also set out for the shape of electric/electronic devices. In the case of plug devices, the standard NEMA 1-15P is set out and applies to plug devices in the US. In the following, the plug size will be referred to as an American type plug.
The plug device is of small physical dimensions, thus requiring special attention to the arrangement of components in order to comply with standards like the IEC standards, however, prior technologies have only managed to partially accomplish an AC/DC power supply integration into a plug-type device to a degree, which is primarily due to the physical size of the converting circuit, which previously tended to be large and bulky.
U.S. Pat. No. 4,273,406 describe an AC/DC power supply accommodated within a plug device. In this power supply device a linear type transformer is retained in a two-part cylindrical casing fastened with a screw. There are several drawbacks of said invention due to its application of a large linear type transformer. The most severe drawback is that the casing is large and heavy. Furthermore, the plug is not easily fitted into the European type sockets.
WO 01/08270 describes an AC/DC adapter of a type similar to U.S. Pat. No. 4,273,406. The invention according to WO 01/08270 fits into European type sockets. However, this is achieved by having a section with the input connectors adapted for insertion into the socket. The remaining components are housed in a large compartment attached to this section.
Presently, most power supply devices adopt switch mode power supply (SMPS) circuits. WO 94/06177, equivalent to German utility model G 9320893U1, describes a European type plug with a built-in power supply. The described SMPS is a Flyback converter. However, the publication does not describe how the SMPS can be arranged inside the plug device and at the same time comply with the required standards.
One of the problems with this type of SMPS is that the creepage and clearance distances must be approximately 5 mm in order for the product to be approved with the safety standards.
Prior art AC/DC power supplies have not been able to exploit the advantages of the switching technology while at the same time feature a compact design which fits into mains European type plug. The two main reasons are that as the dimensions shrink, a new set of problems arrive due to the required primary side to secondary side isolation, and problems in relation to EMC arise due to the switching technology. Several problems within different technical fields will have to be solved simultaneously in order to construct a plug device with small dimensions, preferably of the American type size or even smaller.